WIP: Use go modules to replace $GOPATH based dependencies

[xing-yang]

What type of PR is this?
/kind feature

What this PR does / why we need it:
This PR uses go modules to replace $GOPATH based dependencies.

Which issue(s) this PR fixes:
Fixes #60
Investigate whether using go modules can fix the dependency issue.

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Uses go modules to replace $GOPATH based dependencies.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xing-yang

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[k8s-ci-robot]

@xing-yang: The following tests failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-kubernetes-csi-external-snapshotter-unit	3b28542	link	/test pull-kubernetes-csi-external-snapshotter-unit
pull-kubernetes-csi-external-snapshotter-alpha	3b28542	link	/test pull-kubernetes-csi-external-snapshotter-alpha
pull-kubernetes-csi-external-snapshotter	3b28542	link	/test pull-kubernetes-csi-external-snapshotter

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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